At the present time, as liquid crystal displays, displays of twisted nematic (TN) mode (hereinafter referred to as "TN displays") are most widely used. These TN displays have a number of advantages such as low driving voltage, and low electric power consumption. However, they are markedly inferior in response speed to emissive type displays such as cathode ray tubes, electroluminescence displays, and plasma displays.
Although new type TN displays in which a tilt angle of liquid crystal molecules is set to 180.degree. to 270.degree. have been developed, they are still inferior in response speed to the emissive type displays.
Although various attempts have been made to improve the response speed of TN displays, a TN display exhibiting a quick response has not been developed.
On the other hand, a new display mode using ferroelectric liquid crystals which are now under extensive investigation is expected to be greatly improved in response speed (N. Clark et al., Applied Phys. Lett., 36, 899 (1980)). This display mode utilizes a chiral smectic phase, such as a chiral smectic C phase (Sc*), exhibiting ferroelectric properties. As well as the above chiral smectic C phase, chiral smectic F, G, H, and I phases exhibit ferroelectric properties.
A number of characteristics are required for a ferroelectric liquid crystal material to be used in a ferroelectric liquid crystal device. Typical examples of such characteristics are spontaneous polarization (Ps), a tilt angle (.theta.), viscosity (.eta.), liquid crystal sequence, and so forth.
Among response time (.tau.), Ps, and .eta., there is the following relation: ##EQU1## Thus, in order to obtain quick response, it is necessary that Ps be large, and .eta. be small.
Further, two types of ferroelectric liquid crystal display modes are mainly employed for a ferroelectric liquid crystal display device. One is a birefringence mode using a pair of polarizers, and the other is a guest-host mode using one polarizer and a liquid crystal composition containing dichroic dye. In order to obtain the highest contrast ratio in these display modes, the tilt angles should be 22.5.degree. and 45.degree. in the birefringence mode and the guest-host mode, respectively.
At the present time, however, a compound capable of satisfying the above requirements by itself cannot be found. Thus, ferroelectric liquid crystal compositions comprising a plurality of liquid crystal compounds, or liquid crystal compounds and mesogenic compounds have been proposed as liquid crystal materials for use in ferroelectric liquid crystal displays.
More specifically, there have been proposed: (a) a composition consisting only of ferroelectric liquid crystal compounds; and (b) a composition which comprises a compound exhibiting a non-chiral, tilted smectic phase such as S.sub.C, S.sub.F, S.sub.G, S.sub.H, or S.sub.I phase, or a composition comprising a plurality of such compounds as a base material, and at least one ferroelectric liquid crystal compound or at least one optically active compound which per se is not a liquid crystal being mixed with the base material so that the resulting composition exhibits a ferroelectric liquid crystal phase.
As the base material in the composition (b) above, compounds or compositions exhibiting the non-chiral, tilted smectic phase are used as described above. In practical use, liquid crystals or liquid crystal compositions exhibiting the non-chiral, tilted smectic phase over a wide temperature range from lower temperature to higher than room temperature are desirable. Thus, of compounds exhibiting such non-chiral, tilted smectic phases, those exhibiting an Sc phase are widely employed as the base material, because the Sc phase appears in the highest temperature range among the phases wherein the ferroelectric phase is induced.
As such Sc phase liquid crystals, those such as phenylbenzoate-, Schiff base-, biphenyl-, phenylpyridine-, and phenylpyrimidine-based liquid crystals are used.
As optically active compounds to be added to the above base material to thereby induce ferroelectric properties, a number of compounds have been reported.
However, an optically active compound capable of providing a ferroelectric liquid crystal composition sufficiently meeting the aforementioned requirements has not been obtained.